The photometric output of a spectrophotometer when appearing as an analog signal produced by a photomultiplier must be measured over a background signal containing such elements as photomultiplier dark current, preamplifier offset current and various currents due to thermal changes, stray light, etc. This background is determined by chopping the light beam passing through the spectrophotometer into dark and light segments. The background may then be determined during the dark period of each chopper cycle and subtracted from the total signal during the light period of each chopper cycle to leave the desired photometric signal.
Since the signals being measured may vary over a wide range it is desirable to incorporate a gain control means into the signal processor to maintain the signal level within a range which allows maximum resolution in the analog to digital conversion process used. By combining the background extraction with a subsequent gain controlled amplifier, not only the above cited background elements but also the offsets in the gain controlled amplifier and the analog to digital converter (ADC) can be eliminated automatically without adjustments.
It is an object of this invention to provide automatic gain control to provide the best analog to digital resolution.
It is a further object to provide automatic background correction for the photomultiplier dark current or any change therein due to temperature or excitation voltage or changes thereof.
It is an object to provide automatic background correction for the original offset voltages of the operational amplifiers and the ADC in the photometric circuit.
It is an object to provide automatic background correction for change in the above offset voltages due to temperature change.
It is a further object to provide automatic background correction for changes in the above due to gain changes in the photometric system.